Various types of rotating biological waste water contactors have been used to expose organisms to air to encourage their growth and thereby increase digestion of organic waste materials in water. Representative of such contactors are those disclosed in U.S. Pat. Nos. 3,827,559; 3,894,953; 3,904,525; 3,997,443; 4,083,746; and 4,115,268.
The rotors of the contactors are generally formed by placing discs side-by-side on a shaft, or by spirally wrapping a strip or sheet around a shaft as in U.S. Pat. No. 4,115,268. Various types of spacers are used to keep the discs spaced apart, as well as to maintain the spiral layers apart, and to increase the contact area. The spacers can be separate elements or they can be integrally formed in the discs or spirally wound strip or sheet.
The environment in which the rotors are used is highly corrosive. Because of this, the large size of the rotors and the large number of them needed in waste water treatment plants, it is advisable to make them of an inert, tough, and inexpensive material which can be readily shaped and assembled into the rotors. Commercially available polymeric materials such as polyethylene, polypropylene and polystyrene are representative of those which can be used.
Thissen U.S. Pat. Nos. 4,115,268 and 4,083,746 disclose vacuum thermoforming three dimensional geometrical shapes into thermoplastic polymeric sheet material to thereby form integral spacers in the sheet and the subsequent forming of disc rotors and spiral rotors therefrom. The U.S. Pat. No. 4,115,268 in column 4, line 61 to column 5, line 2, discloses that the spirally wound strip or sheet can be welded together by the use of a heating bar simultaneously engaging the underlying turn of sheet plastic and the bottoms of cup-shaped recesses of a new turn of plastic being applied. It is also stated there that electronic welding can be used. The patent, however, does not disclose how the heat sealing or welding can be done as a continuous operation, nor does it disclose how to apply heat uniformly over an extended area of plastic sheet surfaces to be joined together by a heat seal or welding.